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Magnetic measurements are reported on 3-phase transformer cores assembled from grain oriented silicon-iron, high permeability silicon-iron and laser scribed high permeability silicon-iron. The power loss of each type of material in strip form was measured under controlled distorted magnetisation. The core built from ZDKH had the lowest loss but the deterioration of losses due to building (the building factor) was lowest in the CGO core. The building factor reached a peak in each core at about 1.3T. It was found that the uniformity of the flux distribution and the variation in incremental eddy current loss determine the building factor. At low flux density (<1.3T) the lower incremental eddy current loss of the ZDKH keeps its building factor low. At high flux density (>1.6T) the flux becomes more uniform in all the cores and their building factors drop. The ZDKH material was found to be most affected by the presence of distorted flux in single strip tests but this factor was not predominant in the built up transformers.